1. Field of the Invention
The present invention relates to a technology for adjusting an optical level of a transmission signal to an appropriate value in an optical transmission system.
2. Description of the Related Art
Recently, by using optical fibers as transmission lines, diffusion of optical transmission systems which transmit optical signals that can increase data capacity by being multiplexed by means of wavelength division multiplexing (WDM) at a high transmission rate instead of electrical signals has been remarkable. FIG. 5 is an explanatory diagram of the configuration of an optical transmission system.
An optical transmission system 500 is provided with optical add and drop multiplexers (OADMs) A, B, E, and D and in-line amps (ILAS: optical repeaters) C and F on a transmission line consisting of an outer ring (upward ring) 510 and an inner ring (downward ring) 520. To the respective OADMs A, B, D, and E, transceivers 501A, 501B, 501D, and 501E are connected, and by adding, dropping and transmitting transmission light through the outer ring 510 and the inner ring 520, optical signals can be transmitted to and received from an arbitrary communications partner. The ILAs C and F amplify WDM light to be transmitted through the outer ring 510 and the inner ring 520. The transmission light to be transmitted through the optical transmission system 500 consists of WDM light obtained by multiplexing an optical signal and optical supervisory channel (OSC) light to supervise the optical signal transmission state.
It is important in the optical transmission system 500 that the optical level of an optical signal composing the WDM light is adjusted to an appropriate value by the OADMs A, B, D, and E and ILAs C and F and transmitted through the outer ring 510 and the inner ring 520.
As a conventional technique to adjust the optical level, a structure is available in that in wavelength multiplexing optical transmission, regardless of the optical input level and the number of wavelengths to an optical fiber amplifier, optical outputs equal among wavelengths are obtained and an optical functional part can be inserted in an intermediate portion. In such a structure, to perform generation avoidance of optical surge and judgment of parts connection becomes an important issue. Therefore, feedback control is performed by inserting a variable attenuator in an optical input unit to make constant optical inputs to the amplifying optical fibers. Furthermore, based on wavelength number data obtained from supervisory signals, control to change a total optical output and optical inputs to the amplifying optical fibers is performed, light to the intermediate optical part and light from the optical part are detected, and when no part is detected, excitation is restrained. A technique to avoid the generation of optical surge when connecting and emit signals indicating that an optical part has not been connected yet by performing the control explained above has been disclosed (see, for example, Japanese Patent Application Laid-Open No. H11-17259).
In addition, there is an example in which an optical wavelength multiplexing network is simply constructed. In this case, it becomes an issue that an optical signal level per channel is kept constant and predetermined transmission quality is maintained. Therefore, a supervisory signal transmitted through an optical fiber transmission line is extracted by a WDM coupler and the number of wavelengths of an optical signal to be input to a remote node is obtained from the supervisory signal. The feedback controller calculates the wavelength number data as a sum of the number of wavelengths obtained from the supervisory signal and the number of wavelengths to be newly inserted at the remote node via a supervisory signal processing circuit. Furthermore, a technique is disclosed in that an attenuation amount of the variable optical attenuator is adjusted so that a value obtained by dividing the total optical power of the optical amplifier by the wavelength number data becomes optical power of an optical signal for a desired channel, whereby always feedback-controlling the attenuation amount of the variable optical attenuator and compensating loss fluctuation of the optical fiber transmission line (see, for example, Japanese Patent Application Laid-Open No. 2004-147122).
Conventionally, as shown in Japanese Patent Application Laid-Open Nos. H11-17259 and 2004-147122, optical level control of an optical signal is performed when starting the optical transmission system 500, and an attenuation amount of a reception unit is adjusted based on wavelength number data of the WDM light acquired by OSC controllers installed inside the OADMs A, B, D, and E and ILAs C and F shown in FIG. 5, whereby controlling to an optimum optical level.
An example of a method of adjusting the optical signal level when starting OADMs or ILAs is explained below. FIG. 6 is an explanatory diagram for explaining starting procedures of the optical transmission system. A reception unit 610 includes a front photodiode (PD) 614 and a rear PD 615 in the front and rear of a variable optimal attenuator (VOA) 611. The reception unit 610 and a transmission unit 650 include unit controllers 653. In the reception unit 610, a unit controller 616 adjusts the attenuation amount of the VOA 611 based on optical levels detected by the front PD 614 and the rear PD 615 and controls an optical level of an optical signal to be input into a preamp 613. Furthermore, in the reception unit 610 and the transmission unit 650, the unit controllers 616 and 653 are connected to an OSC controller 660 to adjust the attenuation amount of the VOA 611 when starting.
In FIG. 6, an OR 661 and an OS 662 include a unit controller 663, an optical-electrical converter (OE) 664, and an electrical-optical converter (EO) 665. The unit controller 663 controls the interior of the OSC controller 660. The OE 664 converts an input optical signal into an electrical signal and outputs it. The EO 665 converts an input electrical signal into an electrical signal and outputs it.
Next, starting procedures of the OADM B connected to the outer ring 510 and the inner ring 520 is explained. To start the OADM B, OSC light is transmitted between optical transmission devices adjacent to each other (between OADMs A and B in the example shown in FIG. 6).
First, from the unit controller 663 of the OSC controller 660 of the OADM B, an optical level controlling amplified spontaneous emission (ASE) light output request is output to the unit controller 616 of the OADM A (S1). The optical level of the ASE light requested at this point corresponds to an optical signal 1 wave level. In response to the ASE light output request, to prevent the optical signal from the OADM B from being sent to the transmission line, a 1×2 switch (SW) 617 disposed at the stage before the preamp 613 of the OADM B is controlled to open and shut down the input light to the OADM B.
Next, OSC light communication confirmation is made in the EO 665 of the OADM A and the OE 664 of the OADM B (S2). A postamp 651 that has received the ASE light output request outputs ASE light at a level corresponding to the optical signal 1 wavelength (S3). At this point, to prevent the optical signal from the OADM A from being sent to the transmission line, a 1×2 SW 654 disposed at the stage before the postamp 651 of the OADM A is controlled to open.
When the ASE light is input to the reception unit 610 of the OADM B via the outer ring 510 (S4) and further input into the unit controller 616 via the VOA 611, the VOA 611 is automatically adjusted (S5). Specifically, the unit controller 616 of the OADM B adjusts the VOA 611 to an appropriate attenuation amount by monitoring light receiving power of the rear PD 615 provided at the stage before the preamp 613 so that the input light of the preamp 613 becomes an appropriate level.
When the automatic adjustment of the VOA 611 is ended, the unit controller of the OADM B judges that the input into the preamp 613 has become stable, releases the shut-down state of the preamp 613 of the OADM B (S6), and starts the preamp 613 by means of output constant control (ALC).
When the unit controller 616 of the OADM B confirms that the preamp 613 has started and transited to automatic gain control (AGC), the unit controller stops the ASE light output request for optical level control from the unit controller 663 (S7). When the output of ASE light from the postamp 651 is stopped, the unit controller 653 closes the 1×2 SW 654 disposed at the stage before the postamp 651 of the OADM A, releases the shut-down state of the postamp 651, and starts operation.
The automatic adjustment of the VOA 611 performed at S5 of FIG. 6 means processing to adjust an optical level of an optical signal input in the preamp 613 (ASE light when starting) so as to fall within the dynamic range of the preamp 613.
Herein, the VOA 611 and the dynamic range of the preamp 613 are explained. FIG. 7 is an explanatory diagram for explaining a VOA control when starting the optical transmission system. In FIG. 7, the vertical axis indicates the optical level, and the horizontal axis indicates the time of detection of the optical level indicated on the vertical axis. The PD input time of FIG. 7 indicates an optical level when ASE light is input into the front FD 614 of the reception unit 610. The ASE light to be input into the front PD 614 is controlled to an optical level within a dynamic range (20 decibels) by the optical transmission device of the front stage (OADM A in the example of FIG. 6).
The unit controller 616 adjusts the attenuation amount of the VOA 611 based on detected values of the front PD 614 and the rear PD 615 so that the optical level at the PD input time becomes a target optical level. As the target optical level, an optical level resultant of attenuating 3 decibels from an upper limit of a dynamic range (10 decibels) of the input into the preamp 613 is appropriate. By the adjustment to the target, the attenuation amount of the VOA 611 can be fixed and WDM light at an appropriate optical level can be output from the reception unit 610.
However, as shown in FIG. 7, when an input rise of ASE light for adjustment occurs after adjusting the VOA 611, the preamp 613 does not normally start. FIG. 8 is an explanatory diagram for explaining a VOA control when the input level of the optical signal rises. In FIG. 8, the vertical axis indicates the optical level, and the horizontal axis indicates the time of detection of the optical level indicated on the vertical axis. At the VOA adjusting level El indicated on the vertical axis, the optical level input into the PD is attenuated by VOA, and when inputting it into the preamp 613, the optical level is adjusted so as to fall within the dynamic range of the preamp input.
At this point, when the connection of the connector of the cable to input ASE light is insufficient and the connector is properly connected later, the input level of the ASE light may rise. After being adjusted to the VOA adjusting level E1, when the optical level rises to the final optical input level E2, the VOA attenuation amount is adjusted based on the VOA adjusting level E1, so that when being input into the preamp 613, the AMP input level of the optical level of E2 deviates from the dynamic range.
When the input level of the optical signal input into the preamp 613 is out of the dynamic range, the preamp 613 does not start, or when the preamp 613 is in gain constant control, the output of the preamp 613 rises by following input fluctuation and adversely influences the optical signal.